NodeJs http get downloader with concurrency control

nodejs-get-url-concurrency-control.js

// example
const fetchUrl = fetchUrlFactory(2)
const { content, headers } = await fetchUrl('https://example.com')

// lib
const https = require('https')
function fetcUrlFactory ({ concurrency }) {
  let running = 0
  let queue = []

  function processQueue () {
    if (running >= concurrency) return
    if (!queue.length) return

    const getUrlTask = queue.shift()
    running++
    if (running.length < concurrency) processQueue()

    getUrl(getUrlTask)
  }

  function getUrl ({ url, resolve, reject }) {
    const req = https.get(url, (res) => {
      const { statusCode } = res
      if (statusCode !== 200) {
        reject(new Error(`Unexpected status code: ${statusCode}`))
        return
      }

      let content = ''
      res.setEncoding('utf8')
      res.on('data', (chunk) => content += chunk)
      res.on('end', () => resolve({
        content,
        headers: res.headers
      }))
    })

    req.on('error', reject)
  }

  return function fetchUrl (url) {
    return new Promise((resolveFn, rejectFn) => {
      const resolve = (...args) => {
        running--
        resolveFn(...args)
        processQueue()
      }

      const reject = (...args) => {
        running--
        rejectFn(...args)
        processQueue()
      }

      queue.push({ resolve, reject, url })
      processQueue()
    })
  }
}